#1. Star Wars: Rebellion. Hugely underrated game. Way, way ahead of its time.
#2. Age of Empires (2, mostly, and expansions). Still play this all the time. 
#3. Red Alert.
#4. Starcraft I guess, just because it was really well balanced. It never really felt ground-breaking to me though. Just a solid, gold-standard game. Throw in Warcraft, too, because same reasons.
#5. Total War series.

There's plenty of other good ones. I was fond of 'World in Conflict' because it got rid of build orders, and had a pretty cool dynamic battle system. I do love Sins of a Solar Empire but there's no room on my list.